1. Field of the Invention
This invention relates to a method of rendering non-ferrous metal substrates more resistant to corrosion which involves coating the substrate with a particular type of pigment-free coating composition and curing the same to form a transparent protective coating.
2. Description of the Prior Art
Problems arising from the corrosion of metals induced by the action of acids, salts and other influences such as atmospheric pollution are well known. Many protective coating compositions have been proposed in the past such as coating compositions prepared from monoorganosilanes or hydrolysis products thereof. Examples of such coatings are those taught by Eder et al. in U.S. Pat. Nos. 3,101,277, Burzynski in 3,460,980 and Vincent in 3,061,467. While transparent coatings of the compositions described in the above patents are relatively economical, especially when only methyl trimethoxysilane is used, and do offer a certain amount of corrosion protection, it is desirable to obtain coatings of this type which offer an increased amount of corrosion protection to non-ferrous metal substrates.
The use of metal catalysts which are said to accelerate the condensation of organosilicon compounds has been proposed to increase the hardness, solvent resistance or corrosion resistance of such coatings. For example, Eder suggests that the corrosion resistance offered by coatings of the compositions he teaches can be increased through the use of condensation catalysts such as metal salts of carboxylic acids and the alcoholates of heavy metals such as titanic acid esters such as tetrabutyl titanate. However, Eder requires that the metal be heated to 350.degree. C.-600.degree. C. for curing and the effect of the catalyst at such a high curing temperature appears to be minimal. Sandvig, in U.S. Pat. No. 4,042,749, teaches that cured coatings of a mixture of a reactive silane and a metal ester can be used to render metals corrosion resistant, but requires a silane containing a reactive group such as epoxy or vinyl and teaches away from using completely hydrolyzed mixtures of reactants.
Tetraalkyl titanates are known to be readily hydrolyzed by water and eventually complete hydrolysis of such compounds is believed to lead to the formation of hydrated species of titanium dioxide. Dahle, in U.S. Pat. No. 3,460,956, teaches compositions which are prepared by hydrolyzing tetraalkyl titanates with more than 1.5 moles of water in the presence of lactic or nitric acid in a lower alkanol in which both the reactants and the reaction product are soluble. The compositions contain more than the stoichiometric amount of water, but are described as being clear, water insoluble homogeneous solutions. These compositions are then coated on plastic, glass, steel or aluminum substrates to produce what are described as hard transparent surface coatings after heat curing.
Due to their ability to irreversibly react with moisture, metal alkoxides such as tetrapropyl titanate and aluminum isopropoxide have been suggested by Greyson in U.S. Pat. No. 3,397,046 for use in polysiloxane coatings used to prevent red-corrosion of silver-plated copper conductors. Likewise, Eberius in German OS 1,941,328, suggests the inclusion of organic titanium compounds in solvent solutions of synthetic, halogen-free, water- and acid-resistant binders to protect metallic surfaces such as aluminum and copper with a transparent coating because the titanium compounds can react with corrosive agents in the atmosphere such as sulfuric acid to form colorless reaction products. It would appear that completely hydrolyzed titanium compounds in the form of oxides would not be desirable for use in these applications.
Coating compositions useful in protecting metal substrates from the effects of corrosion are described in British Pat. No. 1,004,893 to the Dow Corning Corporation, in U.S. Pat. No. 3,687,882 to Bishop and U.S. Pat. No. 3,817,905 to Lerner et al. The British patent to Dow Corning is directed to a binder composition containing an organosiloxane resin and 6.5 to 14 percent by weight of a titanium ester. A metallic pigment such as zinc or aluminum is added to the binder to produce a coating composition curable to a corrosion resistant coating useful in protecting metals from corrosion such as that caused by salt spray from the ocean. The binder composition does not appear to contain water nor is anything taught about the ability of the binder itself to render metals corrosion resistant. Furthermore, one would not expect compositions containing metal pigments to be transparent.
The Bishop patent describes a silane-titanate dispersion for coating aluminum to protect the surface of the metal from corrosion. The disperion is prepared by cohydrolyzing a monoorganotrialkoxysilane with a tetraalkyl titanate (usually as a chelate of a 1,3-dioxo compound such as the acetoacetonate chelate of tetraisopropoxy titanate) in a water/alcohol/acid mixture. The solvent mixture is adjusted by removing some of the volatile components by distillation and adding water or an aqueous alkanol solution to the desired solids content. Colloidal alumina can be added to the dispersion when the coated aluminum article is to be adhered through an organic adhesive to another surface. The actual corrosion-resistant coating composition is prepared by adding a hexavalent chromium compound to the above dispersion and applying that mixture to a clean aluminum surface. The silane-titanate codispersion requires a specific type of silane wherein the monoorgano radical attached to silicon by means of a silicon-carbon bond must contain at least one hydroxy radical or other radical such as glycidoxy which will yield a hydroxy radical upon hydrolysis. Bishop does not appear to suggest that the aqueous silane-titanate dispersion itself would have any particular utility in rendering aluminum substrates more resistant to corrosion in the absence of a hexavalent chromium compound.
Lerner, et al., in U.S. Pat. No. 3,817,905, describes a coating composition consisting of a binder composition containing a particulate solid, such as zinc dust, which will impart galvanic protection to metal substrates coated with such a coating composition. The binder composition is prepared by hydrolyzing an organotrihydrocarbonoxy silane such as methyltriethoxysilane in the presence of at least 0.6 moles, preferably 1.0-4.5 moles, of water per hydrocarbonoxy radical, a solvent such as a higher boiling ether or an alcohol and a sufficient amount of an acid to provide a pH of from 1.0 to about 5.5. Furthermore, Lerner et al. teach that a sufficient amount of a hydrolyzable tetraalkyl titanate, or partial hydrolyzate thereof, should be incorporated in the binder composition to improve the physical properties, such as hardness, solvent resistance and adhesion characteristics, of the coating composition. The amount of said titanium compound in the coating composition is said to range from about 0.1 to 5 percent, preferably from 0.5 to about 2.0 percent, by weight based on the weight of the hydrocarbonoxy silane. Furthermore, Lerner et al. teach that the titanium compounds may be combined with the silanes in any suitable manner to form the binder compositions. Example 1 of the U.S. Pat. No. 3,817,905 teaches that the binder composition can be prepared by slowly adding methyltriethoxysilane to a reactor containing water, ethylene glycol monoethyl ether, a very small amount of hydrochloric acid catalyst and tetrabutyl titanate (the weight ratio of silane to titanate being 50:1) and thereafter continuing to agitate the reaction mixture for 2 hours at a temperature of up to 60.degree. C. Then 30 parts of binder was added to 70 parts of zinc dust to prepare a coating composition. When coated on a steel panel and cured, the pencil hardness of the coating was reported to be about 3H while a similar composition prepared in the absence of tetrabutyl titanate had a pencil hardness of about H. The binder prepared in this manner would possibly contain hydrated forms of titanium dioxide particles because the titanate was added to a relatively large excess of water. Examples 2 and 3 teach alternate methods for preparing the binder compositions and no one procedure is described as being preferred over the other. Furthermore, Lerner et al. does not suggest that the inclusion of the small amounts of tetraalkyl titanate taught will improve the corrosion resistance of the coating, and in fact, does not suggest that the binder composition itself, in the absence of the particulate solids, will be useful in protecting non-ferrous metal substrates such as aluminum from corrosion. Also, coating compositions containing particulate solids would not be expected to produce transparent coatings.
In U.S. Pat. No. 3,395,036, Campbell teaches a process for post-finishing pigmented glass fabrics utilizing a post-finish consisting essentially of (a) from 1 to 8 parts by weight of a monoorganosilane such as methyltrimethoxysilane, (b) 0.5 to 4 parts of an organotitanium or organozirconium compound such as tetraethyl titanate and tetraisopropyl zirconate, the weight ratio of (a) to (b) being from 1:1 to 10:1 and (c) 100 parts by weight of water where the pH of the post-finish is from 2.5 to 6.0. Preferably, zirconium compounds are employed. The non-volatile or solids portion of such compositions is rather low, no additional solvent is suggested to stabilize the solution, and no other use for the post-finish composition is suggested.
Stebleton, in U.S. Pat. No. 3,460,975 teaches a process for making tack-free silicone rubber articles for medical use by coating the surfaces of silicone rubber articles with (1) an alkyl titanate such as tetrabutyl titanate, (2) a silane such as methyltrimethoxysilane, or a partial hydrolyzate of (1) and (2), and (3) a volatile solvent such as diethyleneglycoldimethyl ether and curing the coating, but teaches nothing about coating non-ferrous metals with compositions containing colloidal titania to improve the corrosion resistance of the metal.
Pigment-free transparent coatings consisting of an acidic dispersion of colloidal silica in the partial condensate of a silanol which is principally derived from CH.sub.3 Si(OH).sub.3 which are useful as abrasion resistant coatings for plastics and corrosion resistant coatings for aluminum are taught by Clark in U.S. Pat. Nos. 3,986,997 and 4,027,073. While these compositions offer cured coatings possessing high abrasion resistance, the colloidal silica component which contributes to the abrasion resistance is rather expensive and makes the composition less desirable in applications where the high abrasion resistance possessed by such compositions is not required. Paint compositions containing pigments such as titanium dioxide carried in a binder composition of the type described in the above U.S. Pat. No. 3,986,977 are taught by Clark in U.S. Pat. No. 3,976,497. Pigments may reduce the cost of the composition, but can also reduce the transparency of the pigment-free coating compositions.
Schmidt, in U.S. Pat. No. 3,813,363 teaches that organic and inorganic colloidal sols which are stable at an alkaline pH can be added to water soluble cyclic sulfonium zwitterions to produce alklaine aqueous coating compositions which cure to transparent coatings possessing superior scratch resistance when colloidal silica is used. Many other types of colloidal sols are disclosed as being useful including those from water insoluble inorganic oxides and pigments such as alumina, titanium dioxide and the like which are colloidal in size. Nothing is taught concerning the effect of the addition of colloidal oxide particles on the ability of a coating to render non-ferrous metals more resistant to corrosion.
Due to the fact that aluminum articles are strong, lightweight, and attractive in appearance and because the metal itself is recyclable, it is widely being used in place of traditional metals such as steel in automobiles, transportation equipment, ornamental trim articles and cans. It would be highly desirable to obtain an aluminum or non-ferrous metal article which maintains its natural appearance even though it is exposed to potentially corrosive conditions such as road salt or ocean spray.